The present invention relates to an intermittent transmission control system as may be used in a cellular portable telephone system or the like, and more particularly, to an intermittent transmission control system useable in a communication system which involves a burst transmission.
The current digital radio portable terminal such as a personal handy phone system (PHS), which is a personal communications services in Japan, for example, employs 1.9 GHz band as a frequency band, a carrier spacing of 300 kHz, an access scheme which has four channel multiplexed multi-carrier TDMA (Time Division Multiple Access), and a transmission scheme of TDD (Time Division Duplex). With this access scheme and transmission scheme, a signal on a single frequency is divided into eight slots per five milliseconds, for example, with four slots assigned to each of a down-link communication (from the base station to the terminal) and an up-link communication (from the terminal to the base station). This allows four bilateral communication channels to be simultaneously established on a single carrier.
Data is transmitted between the terminal and the base station by utilizing .pi./4 shift QPSK (Quadrature Phase Shift Keying) and a voice coding scheme which includes a compression encoding of a PCM aural signal from a rate of 64 kbit/second to a rate of 32 kbit/second according to the ADPCM (Adaptive Differential Pulse Code Modulation) system.
A carrier frequency is divided into a control and a communication section with the control carrier being always shared by different terminals. As for the communication carrier section, each time a terminal requires a communication, the terminal identifies and uses an empty carrier and an empty slot, and whenever a radio interference occurs, the communication is shifted to another carrier and/or slot to avoid the interference.
For a communication in either a TDMA cellular system as employed in Japan and the United States in which one channel is allocated to each of the down-link and the up-link or a multi-carrier TDMA system as adopted for PHS in which a channel is allocated to a combined up--and down-link, there is a need for a burst control at a given timing upon initiation of a transmission and for a power saving during the non-transmission interval (standby time).
For example, by turning a pin switch within a radio unit on/off at a given timing, a burst control of an output signal from a mixer is achieved, and the output signal is then passed through a transmit/receive switch, a bandpass filter or the like to be radiated from an antenna. In this instance, during the non-transmission interval (or standby time), the gate voltage of a transmission power amplifier is controlled so as to suppress the drain current of a transistor in the amplifier, thus achieving a power saving.
Moreover, it is generally known that a reduction in the power consumption is enabled in the TDMA system by performing a burst transmission on the side of a mobile station only when the sound presence is detected in the sound presence or absence detecting process, i.e., the presence of sound input from the microphone is detected
However, with the conventional intermittent transmission control system as mentioned, there has been a difficulty experienced by a burst receiver of the base station, monitoring the arrival of a burst, in determining whether a failure of arrival of the burst is due to the fact that the voice from the mobile station is silent, to a degradation in the quality of a radio channel (as caused by a fading, the mobile station being located behind a building or a reduction in the level of electric field because the mobile station is far removed from the base station) or a result of failure of resynchronization upon hand-off. This means that the base station takes long time in rendering a determination as to whether a mobile station is still receiving a transmission from the base station.
In addition, a difficulty is sometimes experienced in the reception of aburst by the base station when a silent interval is long lasting. Specifically, there has been a problem that difficulties are presented in achieving AGC (Automatic Gain Control), AFC (Automatic Frequency Control) and a burst location.